A prescription from your GP may allow you to get Viagra or unbranded Sildenafil for a lower cost than over-the-counter Viagra Connect. You can also buy Viagra, generic unbranded Sildenafil , and other licensed ED treatments online, from approved sites, without needing to visit your GP. Viagra vs. Rarely reported side effects include: an erection that will not go away priapism. This site is intended only for U. Similar values for pharmacokinetic parameters were seen in normal volunteers and in the patient population, using a population pharmacokinetic approach.Tell your doctor before taking any over-the-counter supplement. Use caution with any product that claims to be a non-prescription “herbal Viagra.” Some of these products Missing: off the shelf. Headache, flushing, and upset stomach are common Viagra side effects. These effects are usually mild and often resolve on their own. While rare, Viagra can cause an erection that lasts longer than 4 hours (priapism). This can result in permanent damage if left untreated. Vision changes are also possible. Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in adults (Liqrev, Revatio) and children 1 year of age and older (Revatio) with pulmonary arterial hypertension (PAH; high blood pressure in the vessels carrying blood to the .